Heat oven to 200C/180C fan/gas 6. Mix the butter, sugars, egg yolks and vanilla with a wooden spoon until creamy, then mix in the flour in 2 batches. Roll out thinly on a floured surface, then use a standard 30cm ruler as a template to cut the dough into small, even squares. Do this by starting with the ruler flush with one side and cutting along the length of it. Repeat across the width of the dough, then do the same from the top down. Transfer to baking sheets and bake for 8-10 mins until golden.
While the biscuits cool, mix the butter, icing sugar, custard powder and food colouring, if you have any. Pipe or spread a little icing onto a biscuit, then sandwich with 1 or 2 more biscuits. Repeat until all the biscuits are used, then dust with a little more icing sugar.